Data Supplement

Supplement methods -
Definitions of party composition / Similarity of association data obtained from focal
follow versus party follow / The three randomization methods used to extract the PAV
values / Further information on the used models including the results from block and
subset and randomization

Related Data

Article Figures & Data

Figures

Proportion of males as significant associates averaged for each individual over all the three-month periods for males (M) and females (F) in each study community. Each dot represents an individual and the area of the dot is proportional to the number of three-month periods during which a given individual was observed. The darker the dots, the more data points overlay on this value. The horizontal segments indicate the fitted value resulting from Model 1b.

Proportion of three-month periods that a given dyad was significantly associated, separately for each sex combination (for each graph from left to right: females towards females, females towards males, males towards females and males towards males). Each dot represents a dyad and the area of the dot is proportional to the number of three-month periods during which a given dyad was observed. The horizontal segments indicate the fitted value resulting from Model 2b (controlling for maternal kinship and stability of association).

Differentiation (SKPAv) of association values for individuals of a given sex (M and F at the lower part of each graph) towards members of the sex indicated above (for each graph from left to right: females towards females, females towards males, males towards females and males towards males). Higher values indicate stronger skew reflecting fewer top associates. Each dot represents an individual and the area of the dot is proportional to the number of quarters (or years) it was observed. The thin black horizontal line indicates the median of the raw data and the thick black horizontal line indicates the fitted value for each sex combination based on the output of Model 3. The black box indicates the first and third quartile of the data in each sex combination for each community.

Tables

Summary of the seven datasets included in the study. ‘F’ indicates females and ‘M’ indicates males.

number of adult males

number of adult females

% of dyads which were close maternal kin (% of dyads which were mother–offspring)

study site

study community

study species

observation period

median

min

max

median

min

max

F–F

M–F

M–M

Taï

Taï North

chimpanzee

1992–2012

2

1

6

7

2

13

3.1 (3.1)

4.1 (2.7)

1.9

Taï

Taï South

chimpanzee

2000–2012

4.5

2

8

10

6

14

0.4 (0.4)

2.0 (2.0)

0

Taï

Taï East

chimpanzee

2009–2012

5.5

5

6

11

10

12

0 (0)

0 (0)

0

Budongo

Sonso

chimpanzee

2007–2013

14

13

17

26.5

12

33

1.7 (1.7)

2.5 (1.5)

0.4

Kibale

Ngogo

chimpanzee

2003–2004

37

19

38

42.5

15

49

0.5 (0.4)

1.2 (1.0)

0.5

Lomako

Eyengo

bonobo

1990–1998

7

5

8

14

5

17

0 (0)

4.1(3.4)

10.3

LuiKotale

Bompusa

bonobo

2007–2013

7

4

9

13

12

16

0.3 (0.3)

4.8 (4.8)

3.9

Table 2.

Structure of all the models used in the analysis.

sex-top-associates model (Model 1a)

sex-significant-associate model (Model 1b)

what-makes-top-associates model (Model 2a)

what-makes-significant-associates model (Model 2b)

association-skew model (Model 3)

response

sex of the top

sex of the

Was individual 1 top

Was individual 1

skew index of the PAV

associate

significant

associate of

significant associate

distribution (SKPAV)

associates

individual 2 (Y/N)

of individual 2 (Y/N)

fixed factorsa

sex of the individual

species (chimpanzee/bonobo)

species × sex

sex of individual 1

sex of individual 2

species (bonobo/chimpanzee)

Are individuals 1 and 2 kin? (N/Y)

Were individuals 1 and 2 top/significant associate in the previous quarter? (N/Y)

Sex 1 × Sex 2 × species

kin × species

top/significant associate in the previous quarter × species

sex of individual 1

sex of individual 2

species (bonobo/chimpanzee)

Sex 1 × Sex 2 × species

random intercepts

individual ID

community ID

quarter

individual 1 ID

individual 2 ID

dyad individual 1–individual 2

community ID

individual 1 ID

quarter ID

community ID

quarter | individual 1 ID

random slopesb

sex | community ID

sex | quarter

kinship | individual 1 ID

kinship | individual 2 ID

kinship | community ID

sex individual 1 | individual 2 ID

sex individual 1 | community ID

sex individual 2 | individual 1 ID

sex individual 2 | community ID

top/significant associate in the previous quarter | individual 1 ID

top/significant associate in the previous quarter | individual 2 ID

top/significant associate in the previous quarter | community ID

sex individual 1 | quarter

sex individual 1 | community ID

sex individual 2 | quarter

sex individual 2 | community ID

sex individual 2 | individual 1 ID

Sex 1 × Sex 2 | community ID

Sex 1 × Sex 2 | quarter

offset

proportion of males in the community

aSex was dummy coded with females being the reference category; species was dummy coded with bonobo being the reference category; kinship was dummy coded with non-kin being the reference; and ‘top/significant associate in the previous quarter’ was dummy coded with no being the reference category.

bFor inclusion as random slopes we manually dummy coded and then centred (to a mean of zero) kinship, sex and ‘top/significant associate in the previous quarter’.

Table 3.

Results of the ‘sex-top-associate’ (Model 1a) and ‘sex-significant-associates’ (Model 1b) models fitted to test for species differences in the sex combination of top and significant associates. Significant p-values are indicated in italics. p-Values are only given for terms not included in an interaction. Results are based on PAV values derived from the ‘individual randomization’. Results for other randomizations are provided in electronic supplementary material, tables S2 and S5.

Model 1a sex-top-associate

Model 1b sex-significant-associates

sample size

number of top associates across all quarters = 3892, number of unique individual ID = 314, number of quarter ID = 200, number of communities = 7

number of significant associates across all quarters = 17 545, number of unique ID = 312, number of quarter ID = 312, number of communities = 7

null versus full model

χ2

d.f.

p-value

χ2

d.f.

p-value

22.335

2

<0.001

10.779

2

0.005

estimate ± s.e.

χ2

p-value

estimate ± s.e.

χ2

p-value

intercept

0.60 ± 0.27

−0.50 ± 0.19

sex (male)

−0.64 ± 0.47

−0.20 ± 0.36

species (chimpanzee)

−1.50 ± 0.30

−0.54 ± 0.22

sex × species

3.21 ± 0.53

15.41

<0.001

1.52 ± 0.41

7.62

0.006

Table 4.

Results of the ‘what-makes-top-associates’ (Model 2a) and ‘what-makes-significant-associates’ (Model 2b) models fitted to test for species differences in the characteristics of top and significant associates for individuals of each sex. Significant p-values are indicated in italics. p-Values are only given for terms not included in an interaction. Results are based on PAV values derived from the ‘individual randomization’. Results for other randomizations are provided in electronic supplementary material, tables S3 and S6.

Model 2a what-makes-top-associate

Model 2b what-makes-significant-associates

sample size

total number of periods (quarter or year) = 47 845, number of dyads = 7055, number of individuals = 302, number of communities = 7.

null versus full model

χ2

d.f.

p-value

χ2

d.f.

p-value

33.81

6

<0.001

43.33

6

<0.001

estimate ± s.e.

χ2

p-value

estimate ± s.e.

χ2

p-value

intercept

−3.82 ± 0.42

−2.34 ± 0.39

Sex 1 (male)

−0.55 ± 0.28

−0.04 ± 0.15

Sex 2 (male)

−0.22 ± 0.32

−0.06 ± 0.15

species (chimpanzee)

0.05 ± 0.49

0.54 ± 0.46

kin (Yes)

3.67 ± 0.27

21.28

<0.001

3.83 ± 0.58

top/significant associates in previous quarter (Yes)

1.62 ± 0.12

20.99

<0.001

0.74 ± 0.09

15.85

<0.001

kin × species

−1.37 ± 0.68

3.01

0.083

Sex 1 × Sex 2

−0.23 ± 0.48

−0.17 ± 0.27

Sex 1 × species

−0.42 ± 0.31

−0.52 ± 0.17

Sex 2 × species

−0.68 ± 0.37

−0.50 ± 0.17

Sex 1 × Sex 2 × species

2.72 ± 0.52

27.83

<0.001

1.84 ± 0.29

38.31

<0.001

Table 5.

Results of the ‘association-skew’ model (Model 3) fitted to test for species differences in the differentiation of association patterns for each individual towards individuals of the same sex and of the opposite sex, respectively. Significant p-values are indicated in italics. p-Values are only given for terms not included in an interaction. Results are based on PAV values derived from the ‘individual randomization’. Results for other randomizations are provided in electronic supplementary material, tables S4 and S7.

Model 3 association-skew

sample sizes

number of all individuals across all quarters = 7673; number of unique individual ID = 315; number of quarter ID = 200, number of communities = 7; number of quarter ID within individual ID = 3897

In both species, females were more often significant and top associates of other females than males. The species, however, differed for males’ top and significant associates, being primarily males in chimpanzees and primary females in bonobos.

SA

M–F > M–M

F–F > F–M

M–M > M–F

F–F > F–M

controlling for kinship

TA

M–M ∼ M–F ∼ F–F

M–M > F–F & F–M

When controlling for kinship, the sex-specific association pattern holds for chimpanzees, with males primarily associating with other males and females with females. However, the pattern changed for bonobos and neither male nor female bonobos were more likely to have top or significant associates of a particular sex, after controlling for kinship.

SA

M–M ∼ M–F ∼ F–F

M–M > F–F > F–M

association skew

most skewed associations

M → F

F → F

A high association skew indicates that individuals were highly differentiated in their association partners of a given sex, i.e. they associate strongly with a few partners and weakly with the others. Conversely, a low association skew indicates that individuals were little differentiated in their association partners of a given sex, i.e. they associated relatively equally with all individuals. The sex combination with the most skewed (i.e. differentiated) association distribution differed between the two species. In chimpanzees, the most differentiated associations were from females towards other females and the least differentiated association from males towards other males. For bonobos, associations were more differentiated from males towards females compared with all other sex combinations.

least skewed associations

M → M & F → F

M → M

associate in the past

positive effect on association

Individuals' association in the past had a similar effect on current association in both species, indicating that association patterns were as stable in bonobos as in chimpanzees.

kinship

positive effect on association (stronger in bonobos)

Kinship had a positive influence on association patterns in both species, but the effect tended to be stronger in bonobos, indicating that kinship might structure associations more in bonobos than in chimpanzees.